All tea comes from the same plant, be it white, green or black, the difference is created in the oxidization and processing of the leaves once picked. Black tea is more oxidized than oolong, green and white teas, giving it a much stronger flavour.
When tea is picked has a huge bearing on its flavour and aroma. Many teas are picked just once a year, while other teas may be plucked over the course of two or three seasons. Some teas have a main spring crop and a secondary crop in late summer or autumn. Knowing the season that a tea was plucked can reveal important information about what to expect in the flavour and aroma of that tea â particularly important when it comes to the bold flavours of a black tea.
Once a tea plant has reached maturity its leaves can be harvested for many years. Harvests occur in cooler climates or at high elevations to ensure the freshest leaf. Timing is critical, picking must begin at exactly the right point, to ensure that the leaves are large enough, but not too old. Just the top two leaves and the bud are picked for the best black teas.
Once picked the leaves are spread out to wither for up to 18 hours. This is when the leaves lose most of their moisture, becoming soft and pliable, allowing them to be rolled without tearing. Rolling breaks the membranes of the leaves to release the natural juices, which then collect on the surface. After rolling, the leaves are brought into large humid rooms to ferment. The fermentation process creates essential oils from the natural juices. It's these essential oils that give each black tea its characteristic aroma and flavour. The fermentation process must be stopped at the point where the aroma and flavour of the tea have fully developed; this is done by firing the leaves in a large oven. The essential oils dry on the surface and remain relatively stable until exposure to boiling water.
Bricks of black tea were used as currency in Mongolia, Tibet and Siberia into the 19th century. This is because unlike green tea which can lose its peak flavour within a year, black tea can retain flavour for several years.